The present invention relates in general terms to a decoder making it possible to select a predetermined data from a group of P data the latter forming part of a larger group of N data. The present invention is more particularly used in alphanumeric and graphic display systems, but can also be used for data decoding in numerous other cases.
In the following description of the prior art and the present invention, consideration will be given to the case where N=2.sup.n and in which P=2.sup.p, but P and N can be of a random nature if P is equal to or smaller than N.
The present invention is relevant for decoders in the special case where it is attempted to carry out real time decoding, which e.g. excludes the use of counters which sequentially scan a register in order to determine the location of the first or nth "1" contained in said register. In numerous cases in informatics, an attempt is made to find a data forming part of a group of N data. For this purpose, it is appropriate to provide a memory or register containing said N data and addressing said memory. This addressing must correspond to an n bit word, if N=2.sup.n. After having recorded a succession of events corresponding to the successive incidence of the various data, the succession of n bit words corresponding to the data addresses will be recorded. If it is wished to store a large sequence of data, or more specifically data addresses, it is desirable for said addresses to be as short as possible.
There is at least one case where it is possible to shorten the address of the address word to a value below n. This is the case where, during a given sequence, the chosen data is not a random one from among N data, but belongs to a subgroup P of said N data. Thus, during this sequence, it is possible to designate the sought data by its sequence number or its address from among the P data, it is possible to use an address of only p bits if P=2.sup.p. It is then necessary to carry out the correlation between the address or sequence number from among the P data and the address or the sequence number among the N data. In other words, on the basis of the number from among the P data, it is necessary to determine the number from among the N data.
For solving this problem, using conventional and immediate informatics methods, consideration will be given to whether there are 2.sup.N combinations of N data and it is then necessary to use a decoder having a read-only memory or a programmable logic network having a capacity or number of nodes equal to p.n.2.sup.N.